The present invention relates generally to control systems for motor starters, and more particularly to a method and apparatus to economize the function of a motor.
Many electrical mechanical motor starters use bimetallic overload elements to protect a motor during startup and while the motor is running. A typical overload element conducts motor current through it, which in turn causes it to heat up and go into a stressed condition that results in the element changing shape. In some high horse power applications, a current transformer is used to power the overload element. The current transformer provides a current that is proportional to the motor current to cause the element to change shape when the current reaches a certain level. When the motor is turned off, or when an overload trips due to load conditions, the bimetallic element remains in the stressed condition until it cools, regardless of whether or not the motor can be safely started again. When a start command is received and the bimetallic elements are still in the stressed position the motor is prevented from starting. Conversely, once the bimetallic elements have cooled and moved back to their normal position, the overload contacts allow the motor to be started regardless of the temperature of the motor. Also, since the overload elements are still warm from the last trip, the amount of heat that can be tolerated for a specified class of motor is reduced by the amount of heat that is still in the bimetallic element, which can cause the overload elements to become stressed and again change state preventing the motor from obtaining its proper run speed.
In solid state controlled motor starters, once the motor is turned off, or the processor overload trips due to load conditions, the thermistor elements that are located on the current carrying conductors of a motor circuit remain hot after the motor stops. If a restart command is given while the thermistor elements are still sending a high voltage signal due to high thermistor resistance, the processor reacts by engaging the motor protect or disconnect circuits and prevents the motor from starting. If the thermistor components have cooled only slightly, but are still warm due to the amount of retained heat in the motor circuit, they produce a low voltage signal. The processor then disengages the motor protect or disconnects circuits, thereby allowing the motor to start. However, since the thermistor elements are warm due to the amount of retained heat in the motor conductor circuits, a very low amount of additional heat can be tolerated for a specified class of motor start. Thus, the processor will see the thermistor elements again change resistance and prevent the motor from obtaining its proper run speed under load. In order for the motor to obtain its proper run speed for a complete start, the thermistor elements and motor conductors must cool down to the ambient temperature, which typically is longer than necessary for the given class.
Yet another microprocessor overload scheme uses a toroid that provides a signal to various solid state components for signal processing, measuring, and comparing to a reference voltage. The processor reacts by engaging the motor protection circuit and stops the motor when the signal voltages reach an overload condition. However, when a motor is stopped by a stop command or by the processor overload command, the motor is typically quite hot. If a restart command is given, the processor will attempt to restart the motor. The retained heat in the motor increases the internal resistance of the motor windings and thereby effectively reduces the starting current to the motor. The reduced starting current then causes the motor to remain in the inrush portion of the starting curve longer than the processor overload electronics will typically allow and engages the motor protection circuit, thereby stopping the motor prematurely.
These nuisance trips that occur during startup place unnecessary stress on the motor and the load it is driving, and reduces the mechanical life of the overall system. The overload elements and motor components must therefore cool down to the ambient temperature to allow a complete start and avoid such nuisance trips. Additionally, this large inrush for an insufficient time, not only prevents the motor from starting completely, which causes the aforementioned stresses and shortened mechanical life, but also is a waste of electrical power, thereby increasing electric utility bills.
Some attempts have been made to overcome these problems with the use of a fixed time delay scheme to allow the motor controller sufficient time to cool down, but have failed to solve the problem sufficiently. For example, if the cool down time is set for too long of a period, it will delay the time of the next motor start and interfere with production or productivity. If it is set for a too short of a time period, it will cause nuisance tripping of the overload and prevent motor starting, just as in the aforementioned examples.
It would therefore be desirable to have a frequent start protection and economizer scheme that eliminates the need of bimetallic elements and/or fixed time delays, that would be capable of determining if a motor starter system has sufficiently cooled and is safe to start, while not exceeding a maximum number of starts per hour.